Busybox: Bring reboot.c inline with /system/core/reboot/reboot.c

See changes made in:
https://android.googlesource.com/platform/system/core/+/ca8e66a8b0f843812014a28d49208f9f6f64ecbc

Change-Id: I7a1851b65f5abda552357d5c549150848f6b2c67